1. Field of the Invention
The present invention relates to an optical apparatus and vibration type motor driving control method and, more particularly, to an optical apparatus made up of a lens incorporating a vibration type motor for focus adjustment and an apparatus body in which the lens is mounted and which supplies power to the vibration type motor and provides focus detection information, and a vibration type motor driving control method applied to the optical apparatus.
The above optical apparatus is, for example, a single-lens reflex camera having a vibration type motor incorporated in an exchangeable lens.
2. Related Background Art
Of conventional cameras using silver halide films, a single-lens reflex camera, in particular, is capable of exchanging lenses. As such exchangeable lenses, lenses with various focal lengths have been manufactured.
As such an exchangeable lens, an automatic focus control lens is available, which can automatically adjust the focus by driving the focus lens using a built-in motor. This motor is driven on the basis of focus detection information generated by the camera body. As this motor, a DC motor has been used. Recently, however, a vibration type motor has been widely used in place of the DC motor.
This vibration type motor can maintain high torque up to the range of small numbers of rotations, and hence requires no reduction gear. This motor has many advantages. For example, a reduction in size can be attained, and noise can be suppressed.
In the vibration type motor, a driving voltage generated on the basis of a plurality of periodic signals having different phases is applied to a built-in electromechanical energy conversion element to generate travelling waves, thereby rotating a rotating member. The vibration type motor is accelerated by sweeping the frequency (driving frequency) of a periodic signal from a frequency higher than the resonant frequency of the motor to the resonant frequency.
Recently, in a super-telephoto lens having a focal length of 300 mm or more, in particular, a strong demand has arisen for an increase in the driving speed of a focus adjustment motor. This is because such a super-telephoto lens is often used to shoot sports activity such as track-and-field events, and focus control that can faithfully follow quick movements of athletes is strongly demanded.
For driving the vibration type motor at high speed, there is a method in which the sweeping speed of the driving frequency is simply increased. In this method, however, the characteristics of the vibration type motor cannot follow the sweeping speed of the driving frequency, resulting in awkward movements.
In another method, the driving voltage applied to the vibration type motor is raised. FIGS. 1A and 1B are graphs respectively showing the electric power consumption (P) of the vibration type motor and the number of rotations (N) when the driving voltage applied the vibration type motor is doubled, with the ordinate representing a driving frequency f.
Referring to FIGS. 1A and 1B, curves Nx and Px respectively represent the number of rotations and electric power consumption at a general driving voltage value, whereas curves Ny and Py respectively represent the number of rotations and electric power consumption at a voltage twice as high as the general driving voltage.
As is obvious from FIG. 1B showing the numbers of rotations, when the driving voltage is doubled, the number of rotation almost doubles at the same driving frequency.
As is obvious from FIG. 1A, however, when the driving voltage is doubled, the power consumption of the vibration type motor almost doubles as well.
In a single-lens reflex camera, since power is supplied from a battery in the camera body to the exchangeable lens (no battery is installed in the exchangeable lens), the power that can be used by the exchangeable lens itself is limited accordingly.
As shown in FIGS. 1A and 1B, when the driving voltage to the vibration type motor is raised, and the driving frequency is decreased to f1 to increase the number of rotations of the vibration type motor to N3, the power consumption increases to P3. As a result, the voltage of the battery on the camera body side drops. This may cause operation errors in various actuators on the camera body side, resulting in an operation error in the camera.
As described above, if the driving voltage to the vibration type motor is simply raised to increase the number of rotations so as to drive the vibration type motor at high speed, a serious problem arises in the camera system.
According to one aspect of this invention, there is provided a driving apparatus for a vibration type motor which obtains a driving force by applying a periodic signal to an electro-mechanical energy conversion element portion, comprising a setting circuit (e.g., a microcomputer) for setting a driven amount of the vibration type motor, and a power supply circuit for supplying electric power to the driving circuit, wherein when the driven amount set by the setting circuit is smaller than a predetermined amount, the power supply circuit applies to the driving circuit a voltage higher than a voltage to be set when the driven amount is not less than the predetermined value.
According to one aspect of this invention, a vibration type motor driving control method is applied to a driving apparatus including a vibration type motor which obtains a driving force by applying a periodic signal to an electro-mechanical energy conversion element portion, a driving circuit for applying a periodic signal to the vibration type motor, and a power supply circuit for supplying electric power to the driving circuit, where the method comprises the setting step of setting a driven amount of the lens, and the power supply voltage selection step of, when the driven amount set in the setting step is smaller than a predetermined value, applying to the driving circuit a voltage higher than a voltage to be set when the driven amount is less than the predetermined value.